1. Field of the Invention
The present invention relates to an electroplating method for simultaneously plating both a front surface and a back surface of a substrate which has a through-hole vertically penetrating in its interior to fill the through-hole with a metal film, i.e., metal such as copper.
2. Description of the Related Art
A technique of forming a plurality of through-vias of a metal, vertically penetrating through a substrate, is known as a method of electrically connecting layers of a multi-layer stack of substrates, such as semiconductor substrates. It is customary to make vertical through-vias in a substrate by simultaneously plating both a front surface and a back surface of the substrate, which has through-holes vertically penetrating in its interior, to thereby fill the through-holes with a metal film.
As shown in FIG. 1A, a substrate W is prepared which includes a base material 100 having a vertical through-hole 100a defined therein, and a barrier layer 102 made of Ti or the like and a seed layer 104, as an electric feeding layer, which cover an entire surface of the base material 100 including an inner surface of the through-hole 100a. The front surface and the back surface of the substrate W are simultaneously plated to deposit a metal film 106 of copper or the like on the front surface and the back surface of the substrate W and in the through-hole 100a, as shown in FIG. 1B. The metal film 106 in the through-hole 100a has its maximum thickness at a central portion of the through-hole 100a with respect to an in-depth direction (i.e., a longitudinal direction) thereof. Then, as shown in FIG. 1C, the metal film 106 is grown until tip ends of layers of the metal film 106 that have grown from the surface of the through-hole 100a are joined to each other at the central portion of the through-hole 100a. The central portion of the through-hole 100a is thus blocked up by the metal film 106, forming recesses 108 above and below the blocked portion. The plating process is further continued to grow the metal film 106 in the recesses 108 until the recesses 108 are filled up with the metal film 106, as shown in FIG. 1D. In this manner, a through-via made up of the metal film 106 or copper or the like is produced in the substrate W (see Japanese Laid-Open Patent Publication No. 2005-93934).
There has been proposed an electroplating method for filling through-holes defined in a substrate with a metal i.e., a metal film (see Japanese Laid-Open Patent Publication No. 2006-188745). According to this electroplating method, a PR pulsed current, which has a forward current and a reverse current flowing in an opposite direction to the forward current, is supplied to flow between a substrate as a cathode and an anode to fully or substantially fully fill the central portion of the through-hole with the metal.
There has also been proposed a method for preventing whiskers from being produced when plating a printed wiring substrate or the like with copper (see Japanese Laid-Open Patent Publication No. 2008-513985). This method includes the steps of performing a typical forward electrolysis with use of a forward pulsed current using a substrate as a cathode and a reverse electrolysis with use of a reverse pulsed current using the substrate as an anode. The forward electrolysis and the reverse electrolysis are switched alternately during plating of the substrate. Further, during this plating process, a ratio of a duration of the forward pulsed current to a duration of the reverse pulsed current, or a ratio of a density of the forward pulsed current to a density of the reverse pulsed current is changed.
In the case where the through-hole 100a shown in FIG. 1A has a diameter d in a range of 100 μm to 150 μm, the conventional electroplating method can fill the through-hole 100a with the metal film effectively. However, if the through-hole 100a shown in FIG. 1A has a diameter d of as relatively large as 150 μm to 300 μm, it takes a considerable time to block up its central portion with respect to a depth direction (i.e., a longitudinal direction) of the through-hole 100a. More specifically, as shown in FIG. 1C, it takes a considerable time until the surface layers of the metal film 106, growing from the surface of the through-hole 100a, join to each other. Moreover, as shown in FIG. 1D, a void may be produced in the metal film 106 when the metal film 106 further grows in the recesses 108 to fill the recesses 108. Further, if the diameter d of the through-hole 100a is as relatively small as 30 μm to 100 μm, the opening portions of the through-hole 100a may be closed with the metal film 106 before the central portion of the through-hole 100a is blocked up, and as a result, the void may be formed in the metal film 106.